A wire harness for automobile and motorcycle use is formed by binding together a plurality of insulated electrical wires with a cable tie, adhesive tape, or the like. In each insulated electrical wire, a bundle of strands (usually a plurality of strands) made of a conductor such as a copper alloy is covered with insulation. Strands are exposed at a connecting portion (joint) located at the end or middle of an electrical wire bundle, such as a wire harness, and in order to waterproof such a portion, a heat-shrinkable tube or heat-shrinkable cap having a hot-melt adhesive (inner-layer adhesive) layer formed on the inner surface thereof is used. Waterproofing is achieved by placing the heat-shrinkable tube or heat-shrinkable cap over the connecting portion, followed by heat shrinking.
In waterproofing a wire harness, it is required to prevent entry of water from the outside into a connecting portion, and it is also required to block entry of water into the internal interstices between strands (perform water blocking between strands) so that water that has entered from a portion that has not been subjected to waterproof treatment can be prevented from flowing inside the insulated electrical wires in many cases. The reason for this is that water flows in the interior of the electrical wires and may even enter important equipment. However, since the inner-layer adhesive used in the existing heat-shrinkable tube or heat-shrinkable cap has a high viscosity, a process of simply placing and shrinking the heat-shrinkable tube or cap does not cause the inner-layer adhesive to penetrate the interstices between strands, and it is not possible to achieve sufficient water blocking ability between strands (property of blocking entry of water between strands).
Accordingly, in order to achieve sufficient water blocking ability between strands, in existing techniques, before shrinking a heat-shrinkable tube or cap, an operation is performed, such as immersing a connecting portion in a low-viscosity adhesive, or impregnating the interstices between strands in a connecting portion with a thermosetting resin such as an epoxy resin, followed by curing. For example, PTL 1 describes a waterproof structure at a terminal of electrical wires in which a cyano adhesive is made to penetrate and solidify in the interstices between core wires at the terminal of electrical wires, thereby bonding the core wires to each other; moreover, a heat-shrinkable tube having a hot-melt layer (adhesion layer) formed on the inner surface thereof is placed over the terminal, and the hot-melt layer is melted and solidified to perform filling. Furthermore, PTL 2 discloses a water blocking method for a joint electrical wire including a water-blocking material supply step of causing a liquid water-blocking material having a thermosetting property to adhere to a terminal core wire portion at which electrical wires are exposed, and causing the water-blocking material to penetrate the interstices between the core wires; a water-blocking material curing step of curing the water-blocking material with heat generated by resistance welding; and a step of performing water blocking between wires in which a heat-shrinkable cap having a hot-melt applied to the inner surface thereof is placed over the terminal of the coated electrical wires, the heat-shrinkable cap is shrunk by heating, and the hot-melt is made to penetrate the interstices between the coated electrical wires to perform water blocking between the coated electrical wires.